1. Field of the Invention
The present invention relates to an image sensor for reading an image document or the like, and particularly relates to an image sensor with a focus adjusting device.
2. Description of Prior Art
Image sensors are used to read an image document and convert the image document into an electric signal in image scanners, facsimile machines, or the like. The image sensors that are available at present are roughly classified into two types, i.e., an optical image sensor having a size-reduction optical system including lenses for focusing an image document at a reduced scale onto a CCD (charge-coupled device) to read the image document, and a contact image sensor (CIS) having a sensor unit composed of a plurality of sensor elements formed according to a thin-film fabrication process on an insulating substrate which has a width greater than the width of an image document to be read, the sensor unit being held in direct contact with the image document without any size-reduction optical system interposed therebetween. The CIS module is movably guided by two long, spherical railways at both ends thereof. The two long, spherical railways coupled to the CIS module act like a railroad to a train.
A typical configuration of a prior art CIS module 100 is illustrated in FIG. 1. The CIS module 100 is used to integrate all of the components to a compact and light weight module. The CIS module 100 consists of: (1) an LED light source 104 to illuminate the document being scanned, (2) a one-to-one Selfoc Lens Array (SLA) or rod lens array 103 to focus the document image to the image sensor array 106; (3) a hybrid sensing substrate 101 which comprises a plurality of image sensor arrays 106 and its associated circuitry is used to convert an optical signal to an electronic signal; (4) a cover plate 9 where the document to be scanned rests; (5) a connector (not shown) to connect the electronic signal of the module to outside circuitry; and (6) a protective housing 102 to house all of the above components. The LED light source 104 provides red, green and blue light, which combine to provide white light, and directs the light at the document being scanned. The light that is reflected from the document is then gathered by the SLA 103 and directed at the image sensor arrays 106 on the sensing substrate 101 that rests just under the document being scanned. The sensor arrays 106 then record the images by transforming intensities of light that hits the sensor arrays 106 into digital data. The digital data are finally read out by a computer program through the connector to reproduce the document image.
With the development of scanning technologies and increased requirement for high resolution scanning performance, the scanning resolution has been raised from earlier 300 dpi (dots per inch), 600 dpi to 1200 dpi or higher. However, along with the requirement for high optical resolution, focusing has become an important concern during scanning process. In addition, the objects capable of being scanned have become diversified and can be roughly classified into two types, i.e., non-transparent objects and transparent objects. The transparent objects, such as film-based 35 mm slides or negatives, necessitate complicated scanner configurations and high optical resolution requirements. In general, the higher the resolution, the more detail a scanner can capture, which is especially important when enlarging small objects, such as 35 mm slides. High resolution requires low focus tolerance of the optical system. Conventionally, the main measure taken to decrease the overall focus tolerance of the optical system is to strictly control the dimension precision of scanner components during manufacturing. This results in reduced yield rate and increased production cost, and also focus adjusting cannot be implemented. Alternatively, pads of various thickness are used to ensure precise focusing. However, this requires manufacture of a plurality of pads of different thickness that are selected to compensate for the gross tolerance of an end product, so that consistent focus can be obtained for different end products. It is clear that this method complicates the configuration of the end product and the focus adjusting method, and thus has a negative effect on production efficiency and cost.
Hence, an image sensor with an improved focus adjusting device is desired to meet the requirement for high resolution.